Phase sensitive optical time domain reflectometry (φ-OTDR) enables vibration monitoring along the sensing optical fiber by injecting high-coherence narrow-line wide pulse light into the sensing optical fiber and receiving and analyzing the backward Rayleigh scattering signal. It has the advantages of long monitoring distance, high sensitivity, low resource consumption and anti-electromagnetic interference. It plays an important role in the intrusion monitoring and detection of important security sites such as military bases, petrochemical plants, nuclear power plants, airports, subways and prisons.
Vibration signal demodulation is a key technology in φ-OTDR. Nowadays, the vibration signal demodulation methods of φ-OTDR system generally include backward scattered light intensity detection technology and backward scattered light phase detection technology. The backward scattered light intensity detection technology is simple and easy to implement, but is subject to the polarization of scattered light, noise of State system and other factors, and only qualitative detection of vibration signals can be realized. High-precision detection must be realized by backward scattered light phase detection technology. Moreover, according to the theory of φ-OTDR, the phase change of the backward scattered light has a linear relationship with the external vibration signal, so that the accurate measurement of the vibration signal can be realized by detecting the phase information of the backward scattered light.
Coherent demodulation method is a common method for phase detection of backward scattered light in φ-OTDR system. Patent “Distributed Optical Fiber Sensor and Information Demodulation Method” (Application No. 201210099835.8), patent “Phase Sensitive Optical Time Domain Reflecting Optical Fiber Distributed Sensor System Phase Calculation Method” (CN201510941643), and the patent “phase optical time domain reflectometry device and method based on heterodyne detection and phase demodulation” (CN201510245870) all propose to use digital coherent detection technology to receive Rayleigh signal of phase sensitive light time domain reflectometry, thereby demodulating the phase and amplitude of the Rayleigh signal, enabling simultaneous detection of the position, frequency and intensity of the disturbance information. However, digital coherent detection technology requires high-speed data acquisition and processing to ensure signal integrity, which is expensive, and imposes high requirements on both hardware and software, making it difficult to implement.
The patent “a Phase sensitive optical time domain reflectometry phase demodulation system and phase demodulation method” (201611253531.7) proposes a phase demodulation system and a phase demodulation method, which adopts a mixing technique of orthogonal coherent optical signals to realize hardware coherent detection, so that the system has better noise ratio and stability. However, the frequency of mixing frequency of this system is determined by a single acousto-optic modulator (AOM). The frequency of the AOM is usually up to 100 megahertz, so it needs to be realized by using a microwave mixer with a bandwidth of up to 100 megahertz. The method does not actually reduce the processing speed of the hardware and software required by the system, and the implementation is still difficult.